Sex

He wanted to bone her since the first scene. They are happy to be alive after all that happened.

Although, I agree, it was pretty gross when they started going at it with all of that blood and gunk on their faces. I would be like, "No".

Would a duck seriously get hard after all that??

Maybe they wanted to reaffirm that they were still alive after surviving that grueling life-threatening ordeal. After all, sex is commonly perceived as a procreative act and hence a means to remind yourself that you're still alive.

I am the Duke of IMDb bio writers! I am A#1!

She just wanted to get "stung!"

lol at "stung"